Ice Age glaciers brought variety of rock to Illinois

Third grade students in Jen Janik's class at Big Hollow Elementary School in Ingleside wanted to know more about rocks, “What is the most common type of rock in the world? How many types of rocks are there in the world? How many rocks have been discovered in Illinois?”

Rock is our foundation, and it is the outer crust of the Earth that supports us at all times.

Deep under the Earth's surface are huge plates of rock that prop up the continents. Rocks often are confused with minerals — minerals are chemical compounds often found in rock.

All rocks fall into four basic categories: volcanic, sedimentary, plutonic and metamorphic. Within those categories are many divisions that identify the thousands of rock types scattered across the globe.

“Volcanic rocks come from volcanoes and form from lava, molten rock material that cools and hardens at the Earth's surface,” University of Illinois geology professor Brandon Curry said. “Plutonic rocks are like volcanic rocks because they, too, form from molten rock material — also called magma — but in this case they harden or crystallize beneath the earth. Metamorphic rocks are ones that changed from an existing rock, usually by recrystallizing, but aren't volcanic.”

Sedimentary rock is the fallout from hundreds of thousands or years of erosion and compression of rock, plant material, shell and bones to make rock.

Curry gives these examples of rocks you may already be able to identify: “Basalt is volcanic rock; granite is plutonic; sandstone is sedimentary; schist, a shiny mica-rich rock found in mountainous areas, is metamorphic.”

The most common type of rock is volcanic. It can be found on the Earth's surface and under the oceans.

Here in Illinois, there are examples of almost every kind of rock. That's because the Ice Age brought huge glaciers down from southern Canada that swept across the plains.

Once in a while you might come across an erratic — a large granite boulder that was ferried by the glaciers and deposited in a field many hundreds of miles from its original location.

“Illinois has mostly sedimentary rocks at its surface,” Curry said. “The glaciers dragged all kinds of rock types into Illinois. There's a smidgen of shallow plutonic rocks in southernmost Illinois, and granite is buried deep below a cover of sedimentary rocks. Examining a sand and gravel pit in the Chicagoland area, you might uncover all the rock types, although the dominant one will be dolomite, a sedimentary rock that underlies the glacial sediments in northeastern Illinois.”